1. Field of the Invention
The present invention relates to a display device.
2. Description of the Related Art
Organic electroluminescent (EL) elements have been proposed for next-generation display devices and are currently under active development. The organic EL elements are electronic elements that emit light when holes and electrons injected from anodes and cathodes, respectively, are recombined with each other in light-emitting layers. Therefore, the ratio of the recombination current to the total current, that is, the carrier balance factor, is important in improving the emission efficiency of the organic EL elements. In order to increase the carrier balance factor, recombination processes in the light-emitting layers need to be optimized and the injection or transport process of charges from electrodes to the light-emitting layers need to be also optimized.
In organic EL arrays each outputting a plurality of luminescent colors, the mobility of charges moving from electrodes to light-emitting layers, that is, the necessary conductivity, usually depends on the luminescent colors. This is because material properties, such as bandgap and mobility, of the light-emitting layers and the optimum thickness of charge transport layers for an increase in extraction efficiency depends on the luminescent colors. Thus, when the charge transport layers are made of the same material, it is difficult to achieve high carrier balance for each luminescent color.
Co-deposited films made of a charge-transporting material doped with a donor or acceptor dopant are sometimes used as charge transport layers. The co-deposited films can be reduced in driving voltage because the bulk conductivity thereof and charge injection properties of junction interfaces are increased due to the charge transfer between the host (the charge-transporting material) and the dopant. The co-deposited films, which contain the dopant, have an advantage that the conductivity of the co-deposited films can be controlled by varying the concentration of the dopant. As disclosed in Japanese Patent Laid-Open No. 2004-119201, attempts have been made to solve the above problem in such a manner that the concentration of a dopant in each luminescent color is optimized.
In a display device disclosed in Japanese Patent Laid-Open No. 2004-119201, elements are isolated by a separator and therefore all layers including charge transport layers are patterned for each luminescent color. If deposition failure is caused by the misalignment of a deposition mask, a substrate electrode cannot be sufficiently covered with a low-conductivity layer such as a light-emitting layer and therefore an anode and a cathode are short-circuited. That is, a conventional technique for forming charge transport layers by patterning for each luminescent color causes an increased number of failed organic EL elements. This may lead to a significant reduction in productivity.
A technique for continuously providing a charge-transporting material on a substrate in the in-plane direction thereof is known to prevent short-circuiting due to deposition failure. In general, the charge-transporting material used in this technique may have low conductivity. This is because short-circuit currents generated at deposition failure sites can be prevented from flowing between an anode and a cathode and unintended electrical connections between working elements and elements adjacent thereto, that is, side-to-side crosstalk, can be effectively prevented. However, the use of a charge-transporting material with low conductivity causes a problem that it is difficult to achieve high carrier balance for each luminescent color. Since injection properties are lower as compared to a charge transport layer, there is a problem in that an increase in the thickness of the charge transport layer increases the driving voltage of a light-emitting element.